The present invention relates to a magnetic position detection apparatus such as a magnetic encoder, and more particularly to the improvement on an electric signal waveform detected.
There has been proposed an apparatus for magnetically detecting a positional relation and a relative speed of two relatively moving members (or the position and speed of one of the two members relative to the other member), in which a track having magnetized (or recorded) magnetic poles is provided on a surface of one of the two members having a proximate relation to the other member upon the relative movement between the two members and a magnetic sensor including a magnetoresistive (MR) element having its electrical resistance value changing in response to the one member is provided at a location on the other member near to the one member. The intensity of a magnetic field originating from the magnetic poles and applied to the MR element changes in accordance with the relative movement between the two members. This magnetic field intensity is converted by the MR element into an electric signal to detect the positional relation between the two members on the basis of the electric signal and to detect the speed of one of the two members relative to the other member from a change of the detected positional relation.
In the conventional apparatus, a pulse-like electric signal is taken out of an MR element in order to detect the positional relation between two members. However, in order to improve the resolving power in position detection, it is desired that a signal having a sinusoidal form rather than a pulse form is taken out to detect different relative positions at each phase position of the sinusoidal signal, and such a detection apparatus has been proposed. To improve the accuracy of detection in the latter type of apparatus, it is required that a change in intensity of a magnetic field applied to an MR element which change occurs due to a change of the positional relation between the two members is reflected by an electric signal without any distortion. A change in resistance value of the MR element conformable to a change in intensity of the magnetic field applied to the MR element is small or at highest 8%. Therefore, in order that the change in intensity of the magnetic field originated from the magnetic poles in one of the two members is detected with a high output by the MR element on the other member, the MR element must be placed near to the magnetized portion of the one member. In that case, however, when the intensity of the magnetic field applied to the MR element becomes larger than a certain value, the resistance value of the MR element is saturated, thereby producing a distortion in an output signal. In order to eliminate this distortion, not only a plurality of MR elements are combined to cancel any distortion in an output signal detected by the MR elements but also a gap between the two members is adjusted so that the cancellation of distortion is effectively made.
There is a case where it is preferable to use a two-phase or multi-phase signal (for example, two signals different in phase from each other by 90.degree.) rather than a one-phase signal as a signal representative of the positional relation between two members. For example, the use of two signals having different phases allows to detect the direction of the relative movement of two members and to improve the resolving power in position detection. In a typical conventional two-phase or multi-phase apparatus, one magnetized track is provided on one of two member and MR elements corresponding to the respective phases of signals to be detected are arranged in series with each other on a surface of the other member which is opposite to the track. In general, the one or first member is a rotary body having a track on a circumferential surface portion thereof while the other or second member as a magnetic sensor has the form of a plate. Therefore, a gap between the track and a surface of the second member changes along the longitudinal direction of the track. Accordingly, in the multi-phase apparatus, even if a gap between a MR element for one phase and the track is selected to be the optimum value, a gap between a MR element for the other phase and the track may be out of the optimum value. Also, in the case where the MR elements are arranged on the magnetic sensor or the second member along the longitudinal direction of the track on the first member, the eccentricity of the first member or the rotary body if any, causes an inconvenience that even if the optimum gap is established for a MR element for one phase, a gap(s) for a MR element(s) for the other phase(s) deviate from the optimum values. Accordingly, it is difficult to simultaneously establish the optimum gaps for all the MR elements.
In U.S. Pat. No. 4,818,939 assigned to the same assignee of the present application, countermeasure for a third-order harmonic distortion is proposed. However no consideration is made as to removal of a second-order (or even-order) harmonic distortion which remains even if third-order harmonic distortion is removed.
In U.S. Pat. No. 4,774,464 also assigned to the same assignee, a magnetic rotary sensor for detecting absolute position of rotary body is proposed. In U.S. Pat. No. 4,774,464 it is shown that the magnetizings of two magnetic tracks are shifted by P/4, where P is the field pitch. However, it is not shown to remove an even order harmonic component and to also increase the amount of output by constructing MR elements in a bridge structure.